Father, Son and the Holy Spirit

Why is it necessary for the Father, Son, and Holy Spirit to be one substance? Why is it considered doctrinally unsound for them to be separate, perfect entities who share one mind and purpose? Please provide Scriptural evidence for your answer.

Question by a mormon.

The God-head are not substances as you mentioned. They are persons. The bible clearly teaches them to be ONE. They are three in one. The Godhead contains the Father(God) Son(Jesus) and the Holy Spirit. They remain ONE. There is no more to it than that. The rest is a mystery.

Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen. Matthew 28:19-20

For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
1 John 5:7

In the beginning was the Word, and the Word was with God, and the Word was God.
And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
John 1:1; John 1:14

I and my Father are one.
John 10:30